Abstract
Experimental hybridization of X0 and XY chromosome races of the brachypterous grasshopper P. sapporensis did not reveal pre-zygotic reproductive isolation. However, a partial zygotic barrier was found between the X0-standard race from Shimokawa and XY-standard chromosome race from Akan. Approximately 40% of embryos from females crossed with males from other chromosome races developed parthenogenetically, the remaining embryos were normal heterozygotes. Adult F1 males and females from crosses of this type had properly developed testes and ovaries. Non-sister associations and other irregularities in meiosis were not observed in male meiosis. Crossing experiments demonstrated that hybrids between X0 and XY races occur to some extent. The absence of a hybrid zone between the X0 and XY chromosome races may be the result of selection against heterozygotes. Crosses between the XY-Tanno and X0-standard (Teine) subraces resulted in F1 and F2 generations in spite of the many chromosome differences between them such as a X-A translocation and fixed pericentric inversions in four pairs of autosomes. These results do not support the hypothesis that chromosomal differences play a key role in restricting gene flow between the X0 and XY races of P. sapporensis.
Highlights
The initial version of the chromosome speciation hypotheses postulated that chromosome changes are the primary cause of reproductive isolation
As the zygotic barrier between X0 and XY populations has resulted in the parthenogenetic development of embryos (Bugrov et al, 2004), we report the results of a cytogenetic analysis of embryos obtained from virgin females
In all the examples of chromosomal speciation, populations were not absolutely isolated and gene flow may have occurred through narrow hybrid zones
Summary
The initial version of the chromosome speciation hypotheses postulated that chromosome changes are the primary cause of reproductive isolation. Later Shaw and colleagues (Shaw, 1976; Shaw & Wilkinson, 1980; Shaw et al, 1982; Coates & Shaw, 1982) studied another model based on chromosome races in the Australian grasshopper, Caledia captiva. Populations of this species consist of at least three main chromosome races that differ in fixed pericentric inversions as well as in the localization and size of the C-heterochromatic regions. The authors crossed different chromosome races and revealed the role of pericentric inversions in the creation of reproductive barriers These experiments revealed the presence of postzygotic isolating barriers between two chromosome races that were reflected in hybrid sterility and disruptions in meiosis due to cytogenetic and genetic differences in the parental forms. In the bivalents formed by heteromorphic chromosomes, the locations of chiasmata differed from in the parental forms and lead to the collapse of coadapted gene complexes and disruption of the ontogenesis of the progeny (Shaw & Wilkinson, 1978; Coates & Shaw, 1982, 1984)
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